1. Field of the Invention
The invention relates generally to integrated circuit semiconductor devices and more specifically to global positioning system receivers and radio frequency down-conversion of satellite transmissions to pseudo-baseband signals.
2. Description of the Prior Art
Global positioning system receivers use signals received from typically three or more overhead satellites to determine navigational data such as position and velocity. Such systems may also provide altitude and time. GPS signals are available worldwide at no cost and can be used to determine the location of a vehicle, such as a car or truck, to within one city block, or better. Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with the GPS satellites, to generate accumulated delta-range measurements (ADR) from P-code modulation on those carriers and at the same time track L1 coarse acquisition code (C/A-code) to generate code phase measurements. Carrier L1 is positioned at 1575.42 MHz and carrier L2 is positioned at 1227.78 MHz. Less expensive receivers tune only one carrier frequency, and therefore cannot compute for themselves the local ionospheric delays that will appear as position errors. At such carrier frequencies, radio carriers travel by line-of-sight.
The constellation of GPS satellites in orbit about the earth comprises individual satellites that each transmit a unique identifying code in a code multiple access arrangement (CDMA). This allows the many GPS satellites to all transmit in spread spectrum mode at the same frequency (plus or minus a Doppler shift of that frequency as results from the satellite's velocity). Particular satellites are sorted out of the jumble of signals and noise by correlating the received signal to a set of predefined codes that are matched to individual GPS satellites. These codes can be out of phase with the signals. Therefore, "finding" a GPS satellite initially involves searching various carrier frequencies, to account for Doppler shift and oscillator inaccuracies, and search for a code match, using 1023 different code phases and thirty or more code templates.
The limited amount of energy that can be stored and delivered from a battery continues to severely constrain the single-charge field life of portable handheld GPS navigation receivers. Typical units can now operate continuously for only a few hours. The field life can be extended in handheld GPS receivers by packing a relatively larger battery, carrying spare batteries, field-charging the batteries such as by using solar cells, and/or by reducing the current drawn by the electronics.
Conventional battery-saving techniques include the use of inherently lower-power semiconductor technologies, e.g., CMOS, and system management wherein system elements are powered-off when not being used.